This invention relates to electronic distance measuring systems and, more particularly, to an optical range finder for providing precise distance measurements.
Electronic distance measuring equipment is well known and used in a variety of applications, such as for land surveying, map making or wherever measurements of difficult and/or inaccessible locations are required, such as for satellites or use in space. The distance measuring equipment can be applied to use in space for structural dynamic control, measurement of satellite closing distances for docking systems, deployment of antenna structures to precise distances, and other related concepts.
One known type of electronic distance measuring device transmits electromagnetic wave energy toward a target and detects the reflected energy The phase difference between the transmitted and reflected energy is determined and used to calculate distance. This type of "remote sensing" system includes the use of interferometry.
The interferometry techniques utilize a continuous wave optical signal generated by single frequency/coherence in laser devices. A simple technique combines the reflected optical beam with a source reference beam to produce a fringe pattern in which the phase shifts in the propagation path can be "counted". However, for large displacements, the continuous wave optical techniques become error-prone caused by missed counts. These missed count errors are fatal to the system requiring a complete recalibration procedure.
Another known optical range finder described in U.S. Pat. No. 3,778,160 transmits light energy from one location to another and has it reflected back towards the transmitting location. The energy is detected and compared in-phase with the transmitted energy. The phase relationship between the transmitted and detected energy is varied until a predetermined phase relationship exists. The frequency of the transmitted wave energy is then varied until a predetermined phase relationship exists at a different frequency. The difference in frequencies may then be utilized to provide a direct digital display of distance between the predetermined locations. However, this known system operates by performing only two frequency measurements. Therefore, no accounting is made for frequency dispersion errors which occur in the system. For example, phase detector drift, optic detector drift, and any path length drift cannot be accounted for. Further, the two frequencies measured are generated via separate voltage-controlled oscillators which themselves are subject to differential tracking errors.
There is therefore needed an optical range finder which accounts for frequency dispersion errors arising in the system. Further, the system should be capable of calibrating the internal time delay using the difference in frequency between adjacent frequencies.
The present invention meets these needs by providing an optical range finder having a modulated laser with a modulating oscillator frequency locked to a frequency dependent on a calibrated time delay. The laser source illuminates a target device and the time delay introduced into the system causes the frequency of the modulating oscillator to shift. The amount of frequency shift is proportional to the time delay. From this relationship, the precise distance to the target can be calculated.
It is an advantage of the present invention to perform multiple frequency measurements at a multiple number of different frequencies. This allows measurement averaging to occur over different frequencies to eliminate dispersion problems in the system. Further, the system is precalibrated prior to each measurement to eliminate distance drift errors.
It is another advantage of the present invention to provide an optical range finder which can measure distances on the order of 100 feet with a precision of 0.001 inches. The difference in frequency between adjacent lock points of the phase lock loop control system allows for calibration of the internal time delay in the system. By performing multiple measurements, the dispersion of the system's detectors can be characterized to improve the accuracy of the measurements.
The optical range finder of the present invention operates to measure distance by determining the frequencies at which the phase lock loop of the system produces integral number of wavelengths over the round trip distance traveled by the light energy, i.e. from the optical range finder to the target location and back. Any two adjacent frequencies will have only a one wavelength difference. The optical range finder measures the difference between adjacent frequencies, thus eliminating the need to know the absolute number of wavelengths between the optical range finder and the target.
It is a further advantage of the invention to provide an optical range finder operating in both a calibration and measurement mode. In both modes, the optical range finder's control system locks the laser modulating signal to a frequency which produces phase quadrature at the phase detector inputs. In the calibration mode, several adjacent frequencies are measure and the calibration time delay is calculated for each pair of frequencies. This provides a plot of the detector characteristics over the frequency range. In the measurement mode, the modulated laser light signal travels to the target device and back causing the lock frequency spacing to change. Several adjacent frequencies are measured and the measurement time delay is then calculated using the corresponding calibration time delay for the particular frequency.
In this system, two detectors are utilized to minimize the frequency dispersion effects. A separate detector is used to provide the reference signal rather than using the modulating signal as a reference. Further, at the lock frequencies, the phase detector is always at the same fixed operating point. This technique removes the phase detector non-linearity errors. Also, multiple frequency measurements made by the system allows statistical averaging to occur for minimizing random errors in the system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.